Wells associated with the recovery of hydrocarbons are drilled in stages or intervals. At the end of an interval, a steel casing is placed in the hole to support the formation and prevent the drilled hole from collapsing. After a string of casing of one nominal size is placed in the well, a lower interval of the well is drilled with a slightly smaller diameter, and so on. To drill the well, the drilling fluid is circulated from the surface down through the inside of the drill pipe and, up the annulus between the drill pipe and the well bore and thus back to the surface. The circulating fluid carries the drill cuttings to the surface, and serves lubricating and other purposes well recognized by those skilled in the industry. The circulating fluid thus allows the well hole to be efficiently drilled.
The most economical recovery of hydrocarbons maximizes the useful information available to a drilling operator while that operator is making up tubular connections and lowering the drill string into the wellbore. Prior art systems include designs intended to detect "kicks" while drilling a well, and this information is vital to the safety of the drilling operation. A kick is an uncontrolled flow of fluid into the wellbore from the subterranean formation, and typically results from drilling into a zone of higher than expected or unanticipated pressure. Kicks are thus both dangerous and very costly to drillers, and accordingly drilling operators inherently wish to avoid or minimize kicks, or at least detect kicks as early as possible. The early detection of kicks is particularly important in deep water drilling operations. At times kicks may be confused with wellbore breathing, which is a less dangerous phenomenon associated with drilling a well. Wellbore breathing is also referred to by those skilled in the art as wellbore ballooning. As used herein, "wellbore breathing" includes traditionally recognized wellbore breathing and the characteristic sometimes referred to as wellbore ballooning. Wellbore breathing can occur in certain formations and is characterized by the phenomena where fluid is lost to the formation while drilling, then when the pumps are turned off, the fluid subsequently returns to the wellbore. It is important to appreciate that the preferred response by the driller when encountering a kick is almost exactly opposite to the response when encountering wellbore breathing, although only those skilled in the art fully appreciate the significant difference. Accordingly, it is essential that kicks not be interpreted as wellbore breathing and vice-versa. If misdiagnosed, the wrong response will be applied, which will promote rather than cure the problem.
In the absence of a kick, the volume of fluid (excluding minor amounts attributable to drill cuttings and filtration into porous rock) that is pumped into the hole should equal a volume that flows out of the hole, provided that fluid is not being lost to the downhole formation. Drilling mud "pits" are surface reservoirs that the drilling fluid is drawn from and returned to. By monitoring the pit volume, the drilling operator is able to determine any differential or additional flow of drilling fluid from the well. When the pumped circulation of the drilling fluid is stopped, the observed flow out of the well may continue for a short period of time even though the mud pumps are deactivated. Thus the fluid level in the pits may increase for a short period of time once flow is stopped. This information may be misinterpreted as a kick when well bore breathing in fact is occurring, or the information may in fact provide a rapid indication of a kick.
There are two primary types of existing systems for early detection of kicks. One system, which is commonly referred to as the delta-flow method, compares the rate of flow into the well in the drill string to the rate of flow out of the well. An example of the instrumentation commonly used in this delta-flow method is a J-meter. The instrumentation needed to perform early kick detection using the delta-flow method may be both complex, cumbersome and difficult to maintain. Moreover, many variations of this delta-flow method cannot be reliably used on all commonly used drilling fluids since they rely on measured flow rate or measured fluid momentum.
The other commonly used early kick detection method is commonly referred to as the acoustic method. The acoustic method detects density differences in the fluid returning to the surface. The acoustic method typically is not able to reliably detect a water kick since the system inherently relies upon measurement of a significant density difference of the fluid exiting the well compared to the density of the fluid entering the well. In the acoustic method, there is also a lag time between the influx occurring and the detection of the density change on surface. Both the acoustic and the delta-flow methods may be costly.
Prior systems that disclose that the delta-flow methods and/or acoustic method for early kick detection are discussed in the following publications;
1. IADC/SPE 17228, Johnson H. K., "Development and field testing of a High-Accuracy Full-Bore Return Flow Meter"; PA1 2. IADC/SPE 23935, Schafer D. M. et al., "An Evaluation of Flowmeters For the Detection of Kicks and Lost Circulation During Drilling"; PA1 3. SPE 30449, Steine O. G., Rommetveit R., "Full Scale Kick Detection System Testing Relevant for Slim-Hole/HPHT Drilling"; PA1 4. Haines G., Desloovere O. "Driller-Friendly Kick Detector Responds to Small Volume Kicks," Petroleum Engineer International, July 1998.
Prior art techniques also include a method involving visual flow estimation. This technique may provide an indication of an incoming kick. This technique characterizes the flow as "five finger" or full flow, scaling down to no flow (no fingers). This is simply a visual observation made by a member of the drilling crew, and no accurate assessment of the flow rate is possible since the information is simply conveyed in the driller through vocal communication. No baseline data is generated for comparison with the real time data, and real time data is not recorded.
The disadvantages of the prior art are overcome by the present invention. A new technique is disclosed for detecting properties affecting the well and/or the well fluid while drilling. The techniques of the present invention do not have the disadvantages of the prior art systems, and in fact promote a markedly different approach to detect kicks and more reliably distinguish a kick from wellbore breathing.